Electrical power line owners have a desire and need to transmit more power over existing power lines. As the amount of power being transmitted over a given line (conductor) increases, the temperature of the conductor increases. As the temperature of the conductor increases, the sag of the conductor increases. Existing weather conditions, conductor installation conditions and structural characteristics of the line also affect the amount of sag. Excess sag can put the conductor so close to objects on the earth as to cause flashover, which can damage the line and nearby facilities, cause power outages and endanger animal and plant life.
The current method for line re-rating is to combine sag and tension calculations with any one of a number of commercially available power line analysis computer programs, including longitudinal loading models, to calculate the maximum amperage or conductor temperature that can be used for that line which will not violate safety codes or clearance criteria, nor cause structural or conductor damage. If the desired or required conductor temperature cannot be achieved with the existing design, then the power line owner must resort to one or more of the standard current methods of eliminating the clearance violations. These standard methods include: re-conductoring, raising existing structures, re-tensioning the line between dead end sections, or inserting additional structures. The re-rating process may or may not include field verification of the physical parameters of the power line.
Inherent in any power line rating or re-rating process is the need to determine the behavior of the whole power line and individual conductors operating at higher temperatures. The behavior (sag) of the conductors at high temperatures is generally determined by computer programs, such as Alcoa's SAG10, that calculate the tension and corresponding sag of the conductor based on input of environmental parameters, conductor creep, temperature ranges and other factors. SAG10 is generally available and commonly used within the industry.
The behavior of the power line at high temperatures is determined by a longitudinal loading model. Longitudinal loading models calculate the position of the wire after a change in temperature or loading has occurred. The calculations in these models are based upon the length of wire in a span being fixed at a point in time with known environmental conditions (e.g., ambient temperature) and known geometry and physical characteristics (e.g., longitudinal movement of the conductor supports (insulators), type of conductor support, flexibility of the structures, relative elevation of conductor supports and the spans between structures). As the temperature or loading of the wire changes, the length of wire in that span changes based upon the geometry of the span and the physical properties of the wire, such as coefficient of thermal expansion and creep.
Presently, several procedures exist, used individually or in combination, to predict how much power can be transmitted over an existing power line, under given environmental conditions, without violating safety codes or design standards.
1. The Institute of Electrical and Electronics Engineers (IEEE), Electrical Power Research Institute (EPRI) and Power Technologies Inc. (PTI) and others, have developed and modified computer programs which calculate the electrical capacity (rating) of a power line based upon thermodynamics and heat transfer physics. These programs use environmental parameters, such as wind speed, wind direction, ambient temperature, solar radiation and line direction to calculate allowable amperage of a line. "Worst case" environmental parameters are established and the power line rating is calculated. These programs, used in combination with a longitudinal load model or power line analysis program establish the maximum amperage (or conductor temperature) at which the line can be operated without violating clearance criteria or damaging the conductor(s). Exceeding this upper bound will very likely cause clearance violations, damage the conductor, or both. The mathematical formulas found in the power line rating programs are generally accepted within the industry. PA0 2. Devices can be installed on the line to monitor the tension in the conductor. Conductor tension is combined with other environmental data, such as the ambient temperature and solar radiation, to predict the rating or electrical capacity of the line. U.S. Pat. Nos. 5,517,864 and 5,235,861, both by Tapani O. Seppa (and other patents noted therein), relate to methods of calculating the approximate actual sag of an overhead power transmission line by measuring the amount of tension on the line either by tensiometers or swing angle indicators, as well as measuring ambient temperature, both done at two different times, with no power flow, and then remotely transmitting that information to a computer for performance of calculations. From the data received, a Ruling Span can be calculated from which to determine a maximum safe current that can be transmitted by the existing line without creating excess conductor sag. PA0 3. Israel Electric Company, Haifa, Israel has occasionally used selected algorithms to approximate actual sag under certain combinations of conditions of weather, power transmission and physical design; using basic longitudinal load modeling techniques. PA0 4. Power Line Systems, Inc. is understood to have a computer program (SagSec Software by Peyrot of Power Line Systems, Inc. of Madison, Wis.) that performs a mathematical analysis of sag and tension, using longitudinal load modeling, including allowance for longitudinal insulator movement. PA0 5. Multiple power line analysis, pole/tower spotting optimization, profile analysis and display computer programs exist, including TLCADD.TM., PLSCADD, and Optimal. These computer programs are primarily used in the power line rating process to calculate clearance to the conductors. PA0 6. Current methods to increase the amount of power transmitted over a given line generally require taking the line out of service to make extensive modifications to the conductors or structures, or both. An existing conductor can be replaced with a larger conductor to allow the electrical line to carry an increased load. Alternatively, structures in the existing line can be raised or replaced to allow for the increased sag of the existing conductor when carrying the higher electrical load. Also, new structures can be installed between existing structures to eliminate the low clearance areas caused by increased conductor sag.
The procedures of removing small lengths of conductor and of sliding support clamps have been used in other contexts, but have not been considered for the purpose of re-rating of power lines, as analysis, tools and procedures have not been available to perform the complex calculations needed to determine where to make such removals or slide such clamps.